E. Animals: Structure and Function · Animals: Structure and Function

Investigation: How Exercise Changes Heart Rate, Breathing and Blood Pressure

E2.3 E2.1 E3.1 E3.3 A1.1 A1.2 A1.4 A1.5 A1.6 A1.8 A1.10 A1.11 A1.13About 75 minutes

Learning goals

I can measure heart rate, breathing rate and blood pressure accurately and safely, using a stethoscope and a blood-pressure monitor where available.
I can design a controlled investigation of how exercise affects the circulatory and respiratory systems.
I can organize data in tables and graphs and calculate percentage change and recovery time.
I can explain my results using chemoreceptors, the medulla and cardiac output, and evaluate sources of error.

Words to know

resting heart rate
Heart rate measured after sitting quietly for several minutes.

recovery time
The time it takes after exercise for heart rate (or breathing rate) to return to its resting value, or to within an agreed range of it.

stethoscope
An instrument for listening to sounds inside the body, such as heart sounds and breath sounds.

independent / dependent variable
The factor the investigator changes / the factor that is measured to see the effect.

controlled variable
A factor kept the same so it can't affect the results.

percentage change
(new value − original value) ÷ original value × 100.

random error / systematic error
Unpredictable scatter in measurements / an error that shifts every measurement in the same direction.

Your body as the experiment

You have learned how the heart, lungs and blood work. Now you will test them. When you exercise, your muscles suddenly need much more oxygen and produce much more carbon dioxide. Your respiratory and circulatory systems respond within seconds. Measuring that response, and the speed of recovery, is one of the simplest ways doctors and athletes assess fitness. It is also a great chance to practise designing a fair experiment with real data.

Measuring like a professional

Heart rate (pulse):

Use the radial artery (thumb side of the inner wrist) or the carotid artery (side of the neck, beside the windpipe; press gently and only one side at a time).
Use your index and middle fingers, not your thumb (your thumb has its own pulse that can confuse you).
Count for 30 s and multiply by 2, or 15 s × 4 right after exercise, when the rate is changing quickly. A longer count is more precise; a shorter count catches the peak before it falls.

Breathing rate: count chest rises for 30 or 60 s. People change their breathing when they know it is being counted, so a partner should count quietly while appearing to take the pulse.

Stethoscope: earpieces point forward (toward your nose). Place the flat diaphragm on the chest just left of the breastbone for heart sounds; on the back between the shoulder blades for breath sounds. Clean earpieces before sharing.

Blood pressure: a sphygmomanometer (manual or automatic) on the upper arm, with the person seated, rested, feet flat and arm supported at heart level (see the blood vessels lesson).

Safety first

Light to moderate exercise only, such as stepping onto a low step or marching on the spot. Anyone with a heart condition, asthma, an injury or other health condition should check with a parent or doctor first, or act as the recorder. Stop immediately for dizziness, chest pain, unusual shortness of breath or feeling unwell. Have water, a clear floor and a sturdy, non-slip step. Never exercise to exhaustion for a school investigation.

Designing the investigation

Question: How does 3 minutes of stepping exercise affect heart rate and breathing rate, and how long do they take to recover?

Hypothesis: If a person steps for 3 minutes, then their heart rate and breathing rate will both increase, because working muscles produce more CO₂ and need more O₂; chemoreceptors and the medulla respond by increasing breathing and cardiac output. Both rates will then fall back toward resting values within several minutes.

Variables:

Independent: time and activity condition (rest → exercise → recovery). In a class version, you could instead compare exercise intensities (e.g., 15, 24 or 30 steps per minute).
Dependent: heart rate (beats/min) and breathing rate (breaths/min); optionally blood pressure.
Controlled: step height, stepping pace (use a metronome app), exercise duration, body position during measurement (sitting), time since eating, room temperature, who takes the measurements, and how they are taken.

Procedure outline: sit for 5 minutes → measure resting HR and BR (twice; average) → step for 3 minutes → sit and measure immediately, then every minute for 5 minutes → repeat on another day for a second trial.

Why the numbers change

Breathing: working muscles release more CO₂, making the blood more acidic. Chemoreceptors (mainly in the medulla, and in the carotid and aortic bodies) detect this, and the respiratory centre increases breathing rate and depth. Nerve signals from moving limbs also boost breathing at the very start of exercise.
Heart: the cardiac centre increases sympathetic signals, and the adrenal glands release adrenaline. Heart rate and stroke volume both rise, so cardiac output rises.
Blood pressure: in exercise like stepping or running, systolic pressure rises (the heart is ejecting more blood, more forcefully), while diastolic pressure usually stays about the same or falls slightly, because arterioles in working muscles dilate.
Recovery: after exercise, the extra CO₂ is breathed off and the heart slows. Fitter people usually have a lower resting heart rate and a faster recovery.

This is a negative feedback loop: the response (more breathing and blood flow) reduces the stimulus (high CO₂).

The negative feedback loop that adjusts breathing and heart rate during exercise. The response brings CO₂ back toward normal.

Time (min)	0 (rest)	1	2	3	4	5	6	7	8
Condition	rest	exercise	exercise	exercise	recovery	recovery	recovery	recovery	recovery
Heart rate (beats/min)	68	110	128	136	118	98	84	74	70
Breathing rate (breaths/min)	14	22	28	30	24	19	16	15	14

Sample data from one student (average of two trials). Times 1 to 3 were measured during brief pauses in stepping; 4 to 8 while sitting.

The sample data graphed. Heart rate (solid line, circles) and breathing rate (dashed line, squares) both rise during the 3 minutes of stepping (shaded) and then fall during recovery.

Worked example

Example 1: Analysing the sample data

Use the sample data. (a) Find the percentage increase in heart rate and in breathing rate from rest to the peak. (b) Taking "recovered" to mean within 5 beats/min of resting, find the heart-rate recovery time.

(a) Heart rate: (136 − 68) ÷ 68 × 100 = 68 ÷ 68 × 100 = 100% (it doubled).
Breathing rate: (30 − 14) ÷ 14 × 100 = 16 ÷ 14 × 100 ≈ 114%.
(b) Resting HR = 68, so "recovered" means 73 beats/min or less.
Exercise ended at 3 min. At 7 min the HR was 74 (not yet); at 8 min it was 70 (recovered).
Recovery time = 8 − 3 = 5 minutes (to the nearest minute, since readings were taken once a minute).

(a) HR increased 100%; BR increased about 114%. (b) Heart-rate recovery time ≈ 5 min.

Worked example

Example 2: Evaluating the investigation

Identify two sources of error in this investigation, say whether each is random or systematic, and suggest an improvement for each.

Counting for only 15 s after exercise: a miscount of 1 beat becomes an error of 4 beats/min, and errors go both ways → random error. Improvement: use a pulse oximeter or heart-rate monitor, or repeat trials and average.
The subject knows breathing is being counted and breathes differently → may shift every reading the same way → systematic error. Improvement: count breathing while appearing to take the pulse, or use a chest-strap sensor.
Other possibilities: pausing to measure during exercise lets HR drop slightly (systematic: readings too low); stepping pace drifting without a metronome (random); different people measuring (systematic between trials).

Good evaluations name the error, its likely direction or type, and a specific fix.

Hands-on activity

Run your own step test

Grown-up note: see the safety box above. Use the bottom step of a staircase (about 20 cm) with a railing, or a sturdy low step stool. Stop if anyone feels unwell.

You need: a step, a timer, a metronome app (or a song with a steady beat), paper and pencil, a partner, and optionally a home blood-pressure monitor or a homemade stethoscope (cardboard tube or funnels and tubing).

Write your question, hypothesis and variables (use the design above, or change the independent variable: for example, compare 15 and 30 steps per minute on different days).
Draw a data table like the sample one before you start.
Sit for 5 minutes. Your partner measures your resting heart rate (30 s × 2) and breathing rate (30 s × 2) twice. If you have a monitor, take a blood-pressure reading too.
Step up-up-down-down to the beat (for example, 24 complete steps per minute; set the metronome to 96 beats per minute, one foot movement per beat) for 3 minutes. Your partner takes a quick 15-s pulse at the end of each minute.
Sit down immediately. Measure heart rate and breathing rate at the start of every minute for 5 minutes (and blood pressure at 1 minute, if you have a monitor).
Graph both rates against time on one set of axes. Calculate percentage change and recovery time.
Write a conclusion that explains your results using chemoreceptors, medulla, cardiac output and CO₂, and evaluate two sources of error.

What to look for: both rates should rise during exercise and fall afterward, with heart rate usually recovering within several minutes after light stepping. If you measured blood pressure, systolic pressure is usually higher right after exercise while diastolic changes little. Your graph should have a title, labelled axes with units, and a clear key.

Just for fun

In 1816, the French physician René Laennec rolled a sheet of paper into a tube to listen to a patient's chest, and the stethoscope was born. So the first stethoscope was, technically, a piece of paperwork.

Your turn

Why should you take a pulse with your fingers rather than your thumb?

In a step-test investigation, name the independent variable, two dependent variables and three controlled variables.

Why should a partner count breathing secretly?

Explain why breathing rate increases during exercise.

Explain why heart rate increases during exercise.

A pulse is counted as 34 beats in 15 s. What is the heart rate?

Why is this response called negative feedback?

More practice: warm-up

Name two places where you can feel a pulse.

Which way should stethoscope earpieces point?

What is recovery time?

Calculate the percentage increase from 70 to 105 beats/min.

What should you do if you feel dizzy during the activity?

More practice: on your own

Why is a 30-s count more precise than a 15-s count for resting heart rate?

Why might counting for a full minute right after exercise give a misleading "peak" heart rate?

Two students both have a peak heart rate of 140. Student A recovers in 3 min; student B in 7 min. What might this suggest, and why can't you be sure from one trial?

Predict how systolic and diastolic pressure change right after stepping, and explain.

Write a testable question that uses exercise intensity as the independent variable.

Challenge

Using the sample data, compare the shapes of the heart-rate and breathing-rate curves during recovery. Which returns to rest first relative to its own rise? Suggest a reason.

A class pools data: students who reported doing at least 60 minutes of daily activity had an average resting HR of 66; others averaged 74. A student concludes "exercise lowers resting heart rate." Evaluate this conclusion.

Design an improved version of the investigation that a school could run with heart-rate monitors and an automatic blood-pressure cuff. Explain two improvements in precision or validity.

Check your understanding

Quiz

Which artery is easiest to use for taking a pulse at the wrist?

In a step test comparing different stepping paces, the stepping pace is the:

The main stimulus for faster breathing during exercise is:

A pulse count of 22 beats in 15 s gives a heart rate of:

Immediately after moderate aerobic exercise, which change is most typical?

Which is an example of a systematic error in this investigation?

Why should resting measurements be taken twice and averaged?

For parents and teachers

Big idea: E2.3 asks students to use medical equipment (such as a stethoscope and sphygmomanometer) to monitor how the respiratory and circulatory systems respond to exercise. This lesson is a full inquiry: question, hypothesis, variables, data, graphing, analysis and evaluation (A1.1 to A1.13).

How to help: Be the partner! Take pulses and secretly count breathing, run the timer, and look over the graph. A home blood-pressure monitor or fitness watch, if you have one, makes a good extension.

Common difficulties:

Vague variables. "Exercise" isn't enough: name the pace, height and duration.
Graphs without units or a key. Two dependent variables on one graph need a clear key.
Evaluations that just say "human error." Ask: which error, in which direction, and what exact fix?

Safety: light exercise only. Any teen with a heart condition, asthma, injury or other health concern should check with a doctor first or be the recorder. Stop for dizziness, chest pain or unusual breathlessness.